As basic infrastructure created in the 20th Century ages and wears, machinery for rejuvenating or replacing that infrastructure has become more important. While mining and trenching operation machinery may be included in this technology, road milling machinery is, thus far, the most prolific use of the instant machinery.
Road milling equipment utilizes a rotating drum having a plurality of bit assemblies removably mounted on the outside of the drum in spiral or chevron orientation. A typical rotating drum has a bit tip to bit tip diameter of between 44 and 54 inches and includes a plurality of mounting blocks generally secured thereto by welding in spiral or chevron patterns. The patterns noted provide for the bit blocks to be mounted behind and slightly axially to the side of one another such that the bits or combination bit/holders mounted in each bit block may have the tips of the bits positioned in close proximate relation along the axial length of the drum. As such, adjacent bit tips may be positioned anywhere from about 0.200 inch to about ⅝ inch axially apart for either removing concrete, asphalt, or the like, when replacing one or both of the pavement and underlayment for roadways, or may be positioned axially closer together, about 0.200 inch, for micro milling the surface of pavement to remove buckles, create grooves on curved surfaces such as cloverleafs, or the like.
Improvements in the bits and bit/holders that are removably mounted on the bit blocks have increased the useful in-service life of those removable parts. While such bit and bit/holders have been made of steel and hardened materials such as tungsten carbide, the use of diamond coated tips and man-made PCD (polycrystalline diamond) tips, has been shown to increase the in-service life of those bits and bit/holders.
Another improvement in bit/holders has been the invention of quick change holders that have eliminated the necessity of securing such holders with threaded nuts or retaining clips and have utilized the compressive elastic ductility of hardened steel to provide sufficient radial force between the holders and the bit block bores to retain holders mounted in their respective bit block bores during operation. While such bit assemblies have included rotatable and removable bits mounted in bit holders which, in turn, were mounted in bit blocks as noted above, the introduction of diamond materials on bit tips and their increased in-service life has, in some cases, allowed for the combining of bits and bit holders into a unitary construction with the tips no longer being rotatable on the holders.
A need has developed for improved structure at the front leading end or tip end of bit/holders that provide for improved wear characteristics, in-service life and finer milled road surfaces at reduced total cost.